feat: resource caps, Saved Messages, author walls, docs for node bring-up

Node flags (cmd/node/main.go):
  --max-cpu / --max-ram-mb — Go runtime caps (GOMAXPROCS / GOMEMLIMIT)
  --feed-disk-limit-mb — hard 507 refusal for new post bodies over quota
  --chain-disk-limit-mb — advisory watcher (can't reject blocks without
  breaking consensus; logs WARN every minute)

Client — Saved Messages (self-chat):
  - Auto-created on sign-in, pinned top of chat list, blue bookmark avatar
  - Send short-circuits the relay (no encrypt, no fee, no mailbox hop)
  - Empty state rendered outside inverted FlatList — fixes the mirrored
    "say hi…" on Android RTL-aware layout builds
  - PostCard shows "You" for own posts instead of the self-contact alias

Client — user walls:
  - New route /(app)/feed/author/[pub] with infinite-scroll via
    `created_at` cursor and pull-to-refresh
  - Profile screen gains "View posts" button (universal) next to
    "Open chat" (contact-only)

Feed pipeline:
  - Bump client JPEG quality 0.5 → 0.75 to match server scrubber (Q=75),
    so a 60 KiB compose doesn't balloon past 256 KiB after server re-encode
  - ErrPostTooLarge now wraps with the actual size vs cap, errors.Is
    preserved in the HTTP layer
  - FeedMailbox quota + DiskUsage surface — supports new CLI flag

README:
  - Step-by-step "first node / joiner" section on the landing page,
    full flag tables incl. the new resource-cap group, minimal
    checklists for open/private/low-end deployments

node/feed_twonode_test.go

@@ -81,11 +81,11 @@ func newTwoNodeHarness(t *testing.T) *twoNodeHarness {
 	if err != nil {
 		t.Fatalf("chain B: %v", err)
 	}
-	h.aMailbox, err = relay.OpenFeedMailbox(h.aFeedDir, 24*time.Hour)
+	h.aMailbox, err = relay.OpenFeedMailbox(h.aFeedDir, 24*time.Hour, 0)
 	if err != nil {
 		t.Fatalf("feed A: %v", err)
 	}
-	h.bMailbox, err = relay.OpenFeedMailbox(h.bFeedDir, 24*time.Hour)
+	h.bMailbox, err = relay.OpenFeedMailbox(h.bFeedDir, 24*time.Hour, 0)
 	if err != nil {
 		t.Fatalf("feed B: %v", err)
 	}